This invention relates to improvements in the cycle characteristic of charge/discharge of a non-aqueous electrolyte secondary battery.
Recent electronic technologies have conspicuously progressed so that, e.g., miniaturization and/or light weight of electronic equipments can be realized in succession. Followed by this, also for batteries as portable power supply (source), there has been still more increased demand of miniaturization, light weight and high energy density.
Hitherto, as the secondary battery of general use, aqueous solution system batteries such as lead battery, or nickel/cadmium battery, etc. were the main current. These batteries can be satisfied to some extent in the cycle characteristic, but it cannot be said that they have satisfactory characteristic in points of battery weight and the energy density.
On the other hand, studies/developments of non-aqueous electrolyte secondary batteries using lithium or lithium alloy as anode have been extensively carried out in recent years. Such batteries have excellent characteristics of high energy density, small self-discharge and light weight by using, as cathode material, Li contained composite oxide represented by LiCoO2 of high discharge voltage.
However, in non-aqueous electrolyte secondary batteries using, as anode, lithium or lithium alloy, lithium is crystal-grown in dendrite form at the time of charging followed by development (progress) of the charge/discharge cycle and reaches the cathode so that there results the internal short. In addition, since there results production of dendrite, practical quick charge cannot be carried out. For these drawbacks, realization of practical use became difficult.
As a battery which solves such problem, non-aqueous electrolyte secondary batteries using carbon material as the anode, which are so called lithium ion secondary battery, have been remarked. The lithium ion secondary battery utilizes doping/undoping of lithium into portion between carbon layers as the anode reaction. Even if charge/discharge cycle is developed, precipitation of crystal in dendrite form cannot be observed at the time of charging. Thus, such batteries exhibit satisfactory charge/discharge cycle characteristic.
Meanwhile, there are several carbon materials which can be used as the anode. The material which was first put into practical use is coke or carbon in glass form. These carbon materials are material having low crystallinity obtained by allowing organic material to undergo heat-treatment at relatively low temperature, and has been commercialized as practical battery by using electrolytic solution mainly consisting of propylene carbon (PC). Further, in recent years, when propylene carbon (PC) is used as main solvent, graphite or the like which cannot be used as anode has reached usable level by using electrolytic solution mainly consisting of ethylene carbon (EC).
As the graphite or the like, flaky graphite (the flake of graphite such like a fish) can be relatively easily obtained. Hitherto, such graphite or the like has been widely used as conductive material for alkali battery. This graphite or the like has advantageously high crystallinity and high true density as compared to non-graphitaizable carbon material. Accordingly, if the anode is constituted by the graphite or the like, high electrode filling (packing) ability can be obtained and the energy density of the battery is caused to be high. From this fact, it can be said that the graphite or the like is greatly expected material as the anode material.
On the other hand, in non-aqueous electrolyte secondary batteries using the carbon material as the anode, material which can be used as its cathode material is Li contained composite oxide represented by LiCoO2.
However, in Li contained composite oxide, unit crystalline lattice was caused to undergo expansion/contraction at the time of charge/discharge operation such that change takes place in the electrode thickness by about 10% at the maximum. Further, also in carbon material used for the anode, the crystalline lattice is expanded by about 10% at the maximum. For this reason, expansion/contraction takes place in the both cathode and anode electrodes every time the charge/discharge cycle is repeated. For this reason, stress is repeatedly applied to the electrode, so the electrode is apt to be broken. Particularly, in the cathode in which the conductivity of the active material itself is low, electron conducting ability (conduction) is lost by breakage. As a result, deterioration becomes conspicuous. In addition, also in the anode mainly containing metal oxide, there exists similar problems. Therefore, their improvements have been required.
This invention contemplates realizing cathode/anode structure difficult to be broken even if charge/discharge cycle is repeatedly carried out thus to provide a non-aqueous electrolyte secondary battery having long cycle life time and high reliability.
Namely, this invention is directed to a non-aqueous electrolyte secondary battery including anode and cathode consisting of material capable of doping/undoping of lithium, and non-aqueous electrolytic solution in which electrolyte is dissolved in non-aqueous solvent, characterized in that flaky graphite is included as conductive agent in both the anode and the cathode, or in either one of them.
Moreover, this invention is characterized in that granulated carbon having bulk density of 0.5 g/cm3 or more is included, in addition to flaky graphite, as conductive agent.
This invention is characterized in that carbon black is included, in addition to flaky graphite, as conductive agent.
This invention is characterized in that granulated carbon having bulk density of 0.5 g/cm3 or more and carbon black are included, in addition to flaky graphite, as conductive agent.
By allowing the anode and the cathode to include (contain) flaky graphite having high crystallinity and high electron conduction as conductive agent, electrode structure having satisfactory electron conductivity is constituted. Further, by containing granulated carbon having specific material property as conductive agent, granulated carbon holds electrode structure having satisfactory electron conduction. Thus, long charge/discharge cycle life time can be obtained. In addition, by containing carbon black as conductive agent, carbon black functions as electrolytic solution holding material at the active material surface. Thus, longer charge/discharge cycle life time can be obtained.